This invention is concerned with catalysts and methods for reducing the toxicity of exhaust gases from internal combustion engines.
Recently, environmental pollution due to exhaust gases and other emissions from automotive and other internal combustion engines has become a critical environmental problem. No completely satisfactory solution has yet been devised.
The components of exhaust gases from internal combustion engines which are generally considered as the most harmful are carbon monoxide, nitrogen oxides and hydrocarbons which have failed to burn in the engine. Early attempts to avoid these problems were limited to improvements in engine design. More recently, other methods have been proposed, including after-burners designed to complete the ignition of unburned components and catalytic converters designed to convert the toxic components of the gases to non-toxic derivatives.
This invention is concerned more specifically with improved catalysts for use in catalytic converters.
The catalysts presently employed in converters of known types include, for example, oxides of metals, such as iron, manganese, copper, chromium and nickel, and mixtures of these. Noble metals, such as platinum and palladium, deposited on inert carriers, such as alumina and silica, have also been employed.
Two processes are generally utilized with catalytic converters. The first is a two-stage catalytic reaction system in which the exhaust gases exiting the engine are brought into contact with a reducing catalyst to reduce the oxides of nitrogen. The products of this reaction are normally mixed with secondary air and mixture brought into contact with an oxidizing catalyst which oxidizes the carbon monoxide and the unburned hydrocarbons. In a second or single-stage system, the exhaust gases are typically mixed with air to adjust the oxygen concentration and the mixture is brought into contact with a catalyst for simultaneous reduction of nitrogen oxides and oxidation of carbon monoxide and unburned hydrocarbons.
The single-stage process is generally preferred since it can be implemented in a relatively small catalyst vessel so that design problems for installation are alleviated. Additionally, the exothermic and endothermic reactions proceed simultaneously so that overall conversion may be conducted with a high degree of efficiency. Amongst the disadvantages of the single-stage process are that with conventional catalysts, the range of oxygen concentrations for satisfactory reaction is critical and difficult to adjust, and the simultaneous catalytic oxidation and reduction reactions require the use of separate catalysts and the presence of these in one reaction vessel may result in a reduction in the efficiency of each of them.